This invention relates to method and apparatus for detecting the presence or absence of a coating on the surface of a substrate. In particular this invention is directed toward evaluating the efficiency of coating processes on sheet-like materials such as paper, cloth, polymer films or the like, utilizing the principles of frustrated total internal light reflection.
The phenomenon of total internal light reflection has been long known in the field of optics. Concisely stated, this phenomenon is the observation that when a beam of light is directed onto a first end surface of a longitudinally extending medium having a relatively high refractive index and surrounded by a second medium having a relatively low refractive index, the light does not pass through the longitudinal interface between the medium but instead is reflected back into the first medium. Ultimately, through successive total internal reflections, the light passes through the first medium along its longitudinal length and out the end remote from the first end. Said in other words, the light is trapped essentially totally, within the first medium and does not escape from first medium along its longitudinal interface. It is also known that in the process of total internal reflection, some associated electromagnetic field does escape the interface in the form of an evanescent, nonpropagating field, normal to the medium interface, the aplitude of which decays rapidly into the second medium, at an exponential decay rate with the distance from the interface. This principle is described and applied in "Internal Reflection Spectroscopy", N. J. Harwick, Harwick Scientific Corporation, Ossing, N.Y. (1979).
In the above referenced publication, the principle of frustrated total internal reflection has been suggested for employment in various applications such as, for example, an inkless finger printing apparatus.